1. Technical Field
The present invention relates to digital radiographic detectors. More particularly, the present invention relates to a digital radiographic detector with two scintillating screens and improved spatial sampling.
2. Description of Related Art
Digital radiographic imaging devices are becoming increasingly used in both medicinal and industrial applications. Conventional radiographic imaging devices generally include an array of pixels formed on a substrate. Each pixel includes a photosensitive element and a readout element. Conventionally known photosensitive elements include p-n junction photodiodes, metal-insulator-semiconductor (MIS) photo-capacitors, and pinned photodiodes, among others. The presently most commonly used readout element is a thin-film transistor (TFT), although other readout elements also may be used. In use, the photosensitive element converts an incident light into an electrical signal, and the electrical signal is read out by the readout element. An electrical signal for each of the pixels is read out, and these signals are used to recreate the image, across the array. For example, using appropriate processing, the electrical signals can be displayed on a display or video monitor, to show the exposed image.
Digital radiation detectors are conventionally used with an X-ray image source. Specifically, an article to be imaged, such as a person or inanimate object, is placed between the X-ray image source and the radiographic imaging device and the article to be imaged is exposed with X-rays. The X-rays pass through the article and are detected upon their emergence from the article by the radiographic imaging device. The X-rays may be detected or may first be converted to visible light by a scintillator. When a scintillating screen is provided, it is usually placed between the article and the photosensitive element, to convert the X-rays to light in the visible spectrum, for conversion to an electrical signal by the photosensitive element.
Generally, medical X-ray detectors employing a scintillating phosphor screen to absorb X-rays and produce light suffer the loss of spatial resolution due to lateral light diffusion in the phosphor screen. To reduce lateral light diffusion and maintain acceptable spatial resolution, the phosphor screens must be made sufficiently thin.
The spatial resolution and X-ray detection ability of an imaging apparatus are often characterized by the modulation transfer function (MTF) and X-ray absorption efficiency, respectively. Thin phosphor screens produce better MTF at the expense of reduced X-ray absorption. Usually, the coating density and the thickness of the phosphor screen are used in the design tradeoff between spatial resolution and X-ray absorption efficiency.
For example, the Lanex Fine and the Lanex Fast Back screens are two typical commercial screens manufactured by Eastman Kodak Company, both made of Gd2O2S(Tb) phosphor. The Lanex Fast Back screen is thicker and absorbs X-rays more efficiently, but has lower resolution than the Lanex Fine screen. On the other hand, the Lanex Fine screen is thinner than the Lanex Fast Back screen, absorbs X-rays less efficiently, but has higher resolution. The coating densities of the Lanex Fine and the Lanex Fast Back screens are 34 mg/cm2 and 133 mg/cm2, respectively. The Lanex Fine and the Lanex Fast Back screens have X-ray absorption efficiencies of 24% and 63% (for 80 kVp, with tungsten target, 2.5-mm Al inherent filtration, and filtered by 0.5-mm Cu) and an MTF values of 0.26 and 0.04 at 5 c/mm, respectively. In general, the signal-to-noise ratio (SNR) of an X-ray scintillator increases as the X-ray absorption efficiency of the scintillator increases. The MTF of an X-ray scintillator can also be evaluated by the spatial frequency at which the MTF equals 50% (f1/2). As this spatial frequency (f1/2) value increases, the MTF of the scintillator also increases. For the aforementioned example, the value of f1/2 is 2.6 c/mm for the Lanex Fine screen and 1.0 c/mm for the Lanex Fast Back screen.
In order to improve X-ray absorption and maintain spatial resolution, the use of double screens in conjunction with a double-emulsion film has been incorporated in conventional screen-film (SF) radiographic apparatuses. Similarly, the dual-screen technique has also been used in computed radiography (CR) to improve the X-ray absorption efficiency. In a digital CR apparatus, a storage phosphor screen is used in place of the prompt emitting phosphor screen employed in the SF apparatus. No film is needed. Upon X-ray exposure, the storage phosphor screen stores a latent image in the form of trapped charge that is subsequently read out, typically by a scanning laser beam, to produce a digital radiographic image.
Another imaging technique, known as dual energy subtraction imaging, has been used to reduce the impact of anatomic background on disease detection in chest radiography and angiography. This method is based on the different energy-dependent absorption characteristics of bone and soft tissue. In general, two raw images are produced. One is a low-energy and high-contrast image, and the other is a high-energy and low-contrast image. By taking nonlinear combinations of these two images, pure bone and soft-tissue images can be obtained. This imaging technique would improve diagnosis of pathology and delineation of anatomy using images.
The dual energy subtraction imaging method has two general approaches: dual-exposure technique and single-exposure technique. In the dual-exposure technique, two different images are obtained from a detector by making two exposures at two different X-ray tube voltage settings. Since a double exposure of the patient must be performed, and the switching of the X-ray tube voltage must take a finite time, the double exposure technique would be sensitive to patient motion artifacts and to misregistration between the two images. In the single-exposure technique, in which an energy filter is sandwiched between two detectors to attenuate the low-energy component, two different images are simultaneously obtained by making only one exposure of the patient. The single-exposure technique has the advantages of reducing patient motion misregistration artifacts and reducing X-ray dosage. The dual energy subtraction imaging has been implemented in both the screen-film and computed radiography apparatus with either the single-exposure or the dual-exposure technique.
While digital radiography has brought X-ray imaging into the digital age, and several improvements have already been made in this field, the technology has not yet been optimized. For example, by increasing the signal-to-noise ratio of output of each pixel, a better representation of the imaged article can be obtained. Better images also can be obtained by increasing the spatial frequency and the modulation transfer function. However, and as will be appreciated by imaging designers and manufacturers, these factors that increase the efficacy of imaging detectors are often at odds with each other, that is, taking steps to improve the signal-to-noise ratio often leads to worse spatial frequency or modulation transfer function.
Thus, there is a need in the art for an improved imaging apparatus. Specifically, there is a need in the art for a radiographic imaging apparatus with improved signal-to-noise, spatial frequency, and/or modulation transfer function characteristics.